Learning Forum

[Francesca_Pistorio]

Hello everyone!

I have a doubt and I hope someone can help me solve it. I am modeling a MEMS gyroscope in ANSYS APDL for my Master's thesis. The initial design is very basic and it is only necessary to get an initial idea on how to simulate more complex designs.

My structure is composed by a central mass, suspended by four beam. Since the tickness of the central mass is small compared to other dimensions, I modeled it with shell181 elements, while beams are modeled using beam4 elements. Since these elements share the same DOFs, can I couple them simply sharing the node at the interface? I ask this beacause I have read on the section 2.3 "Limitations on Joining DIfferent Elements" of ANSYS APDL 2019 R3 guide that it is inconsistent to connect only one node of a 3-D beam element to a 3-D shell element such that a rotational DOF of the beam element corresponds to the ROTZ of the shell element.

Moreover, I performed a modal analysis in order to find eigenfrenquencies of my structure. I noticed that, if I increase the central mass elements number, I obtain a results lower than expected, while with a less fine mesh, my result is near the analytical one. I don't know if this is linked to the interface between shell and beam elements.

I share a photo of my model with the coarse mesh (which gives me good results) and my APDL code.

Thank you for the help.

Kindest,

Francesca

[Hui Liu]

Is there any specific reason that you are using the legacy beam4 element? I'd recommend using our current technology beam188/189 elements. Shell181/281 has 6 DOFs so merged nodes are allowed. Does the mode shapes look correct in your modal analysis? Alternatively you can try fixed joint connection between beam and shell.n

[Francesca_Pistorio]

Dear nI solved the problem using beam188 elements (but it's the same with beam4 elements) by using constraints equations.nnThank you.n